Resistance materials and elements



Sept. 2, 1952 F. R. QulNN 2,609,470

RESISTANCE MATERIALS AND ELEMENTS Filed July 22, 1949 mgl. @2.2

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Frederic Q. Qui11T1,`

10 mirc l TEzgEcliA-rvnc. HS Attorney.

'Patented Sept. 2', 195.2

UNITED comici Frederic LR; Quinn,y Schenectady, N. Y., assignor toI GeneraliElectric- Company, i,afcorporation: of

` New-York 'I'he "present .invention relates ,to resistance materials .and `elements `and .more particularly to metal -sl'li'de resistance ,materials and elements.

lThe `invention fis ,predicated ,one the discovery that ,resistance elements .and vmaterials characterzed by ahigh degree of .chemical stability vand stability of resistivity canbe .obtainedby utilizing-,compositions containing as essential .ingredients atleast two dilerent metal subtitles, 2including .molybdenum sulfide-,and a.slde o'fla metal of ,group III rof ktheperiotlic table.

".'It hasbeenv known that Yvarious ,compounds .of metalsjmore speci'cally the ,oxides,.su1`des, and seleni'des, or mixtures thereof possess `certain desirable electrical resistance characteristics which make them lparticula'rly attractive jfor various applications in which .a semi-conducting material is required. Asmany ofthe compounds or mixtures "have Aalso .been characterized by La resistance` which' varies'with vchanges intemperature,'they have Ybeen used in Aregulating and protective "systems "as, "for exa'n'iple,Y for "temperature "control purposes However, "the sulde compositions "known "heretofore have been '..relatively -unsatisfactory 'due 'to their "inherent instability,particularlywhen'subjected to. elevated temperatures.

Oneobje'ct or'thepresentfnventiorrist'ojprovide stable @sulde l resistancematerials "and elements.

*Another object 4of the "invention", is "to provide metal "sul'de resistancematerials and elements which *i are *characterized by"a "high Idegree "of chemical *stability vand stability of 'resistivity whensubjectedfto Vwide' variationstin' temperature.

A further*object offthe present invei'itio'n kis to` provide al process yforl stabilizingI theelectrical characteristics of l*metal sulfide resistance elements.

Other objects andfeatures of the present invention include Iadditions "for Venhancing vor irnproving the properties :of the elements.; such Vas additions of `Lfree "sulfur, "selection "of "suitable leads, ythe `preparation 'of 'the "elements and the treatment'of' thev elements `by "a-process designed to Vstabilize their electrical characteristics particlarly for use'in directcurrentcircuits. These specic Vfeatures are hereinafter described, 'and further illustrated by'- examples 4ci the i preparation of lcertain elements, which examples @are intended'A to setl forth 'more specifically howth'e various `-aspects 'of the present "invention 'are carried into e'ect.

The above and additional objects Iandv features will become apparent "from the following description taken lin connection with the accompanying .drawing v`wherein Figs. v:1-5a .inclusive tare 2 cross-se'ctionalfv-iews annif-Fig. 25h an end fviewaof various -lorms of elements: 'embodying f thefpresent invention; and Figs.f68"inclusive illustrate lthe temperature-resistance:characteristicsobtainable in certain-of the=productsvofflthe present invention.

While 1 the elements of the .present l invention may be vprovided infanyfgof aznumberraofizforms, a simple :"forme. oifelementzo'f fthe thermis'torttype -isnish'ownin Fig. 1l fiinuwh'ich snumeral -.I indicates a body of lairnixture 'fof metal .sull-idas :including at least one of the group IL-'metalsuldeswith the ends ofthe ,leads .2 tand,3 :embedded in opposite ends ofthe body l.

A modificationofftherelement iofaFig. .l'isshown in wherein :the sulfide .f `looidy f4 is enclosed within fand: sealed into a.: :glass Lhousing 515 T:with theleads 2 6 Vand 5:1 extending :through the voppositeends: of' the ,'housing. lhezenvelopen: provides-both physical :and @chemical protectionrfor the. suldeelement 4.

l,=In.Fig.3,`:the electricalf.icontacts 8 :and :asa-re shown intherform-` ofacapsizwhichtmayzbe pressed on to the sulfide :body flz `during the :pressing operation. lThese Acapf'shaped L:cont-acts.iencompassing portions 'sof ftheesuide :elements I:arriere-- spectively :connected to lead A:wireshll :and 5.212 which may :be zany ':electrical tconductor,fssuchws a copper or .nickel Vgwire, soldered :or fweldeldror otherwise ,suitably connected .to-f the-:contacts 1:8

fand? 9 .In fFig. there `is shown a self-contained 'element rin which one :of .lthefscoontacts f.-|A.1;is;;in the 1 form-:oi ya ,case -with the :sulfide ,composition lipressedn in thewcase andftsinteredeinfjsitu. Jn this modication, 'f the `secon-d 'lead i lwgis intthe form `of afwire yhaving :za -attened end portion I1 t0 provide `additional anchoring 'zmeans :for the lead and increased contact: surfacezwit-hrthe sulde ymixture .-l-5. fThe vupper or iopenffend of thev casing I4: can Joe-sealed; by, meansoianysuitable sealing-material t t9. I Al heat hardenablesilicone resinfiis particularly useful for.; this-,purpose both ybecause Hof .its temperature l. resistance @and because of theiact :that zit for-ms a :fheat: resistant -glassy lexterior surface Electrical contact may be. made .directly with the :case l 4 gor fby meansy of a z suitable :lead |18 .fconnected-itherfeto.

An` velementaof .the `designshown r.in rFgS. @5.a and; 5b is especia-ily: suited. lformally applications wheresa quick .temperature responseigparticularjly to :radiant energy, is required. .In Y this @element the sulfidabodyeZ is generallyf atbut is provided with'enlarged end;` portions 2l about leads 22 Lto 4,obtain the desired conductivity rada' acent theKVV contacts. FDue to thevahigh :ratio cof igsurface area to total mass, this modification has a relatively quick response to any change in temperature of the surroundings.

Sulfdes The present invention is primarily based on the discovery that pressed, and preferably sintered, mixtures of metal sulfides essentially including at least one sulfide of a metal of group II of the periodic table, particularly a sulfide of magnesium, calcium, cadmium, zinc, barium or mercury, are characterized by a time and temperature stability much greater than exhibited by any of the known metal sulfide compositions. More specifically, the products of the present invention comprise a resistance or semi-conducting body consisting essentially of at least one group II metal sulfide hereinafter referred to as the essential sulfide, and at least one additional metal sulfide referred to hereinafter as the base sulfide or sulfides. The base sulfide may or may not be a group II metal sulfide. In addition to the essential and base sulfides, the resistance bodies of the inventionalso preferably include small amounts of molybdenum sulfide or sulfur or both. Further aspects of the invention include .the combination of the sulfide bodies With contacts or leads of a particular type and the processing of the combination to provide an element having stable electrical characteristics for both A.C. and D.C. applications.

Essential and base sulfdcs Ordinarily the essential sulfide is present in an amount ranging from about 3 to 95 percent, .generally at least about percent, by weight of the sulfide mixture. The preferred content of theessential sulfide is dependent upon the particular suldes employed in making the elements. The primary function of the essential sulfide is to provide a composition which is stable and will exhibit the same resistance characteristics, i. e., will have the same resistance at any given temperature, over long periods of time or when repeatedly subjected either to sub-zero temperatures or heated to elevated temperatures up to and including those temperatures at which the single sulfides or mixtures of sulfides not containing one of the essential suldes change their resistance characteristics generally as a result of the release or liberation of sulfur. Not only are the electrical characteristics of the products of the present invention stable, but they are also reproducible from batch to batch. The sulfur of the sintered polysulfide mixtures or alloys does not appear to be liberated even when the temperature of the products is raised to or above the evaporation temperature of sulfur. f

` All of the compositions and'electrical elements of the present invention have resistance characteristics which change markedly with change in temperature. Practically all of them have substantial negative coefficients of electrical resistance. By proper selection of the alloying sulfides and the proportions thereof, products may be prepared which either havea constant or substantially constant decrease in resistance with increased temperature or which exhibit a sharp break in electrical resistance at a certain critical temperature, or narrow range of temperatures, the break-point temperature depending upon the composition of the metallic sulfide mixture and to a lesser extent upon the particular process employed in making the resistance element. Examples of sulfide mixtures which in certain proportions exhibit such sharp breaks in their resistance-temperature curves are silver calcium sulfide with a break temperature of around 175 C., and copper calcium sulfide with a break temperature slightly below C. Molybdenum calcium sulfide mixtures can be prepared having break temperatures anywhere from 250 to 400 C. rdepending upon proportions used.

For any given application, the choice of a particular essential sulfide or sulfides will depend upon a number of factors. When the product is to operate at temperatures less than 500 C., the generally less lexpensive calcium, cadmium or magnesium sulfides are preferred. Calcium sulfide has the further advantage in that mixtures including it have very low temperature coefficients of expansion. If higher temperatures are to be encountered, the sulfide of mercury, and particularly the sulfides of zinc or barium should be used since polysulflde compositions containing these essential sulfides are stable at temperatures up to 1200 C. Cadmium sulfide compositions in general exhibit comparatively sharp break points, and steep temperature-resistance curves With calcium sulfide ranking next to the cadmium sulfide in this respect. Calcium sulfide exhibits no break temperature of its own so that the temperature-resistance characteristics, and particularly the break-point temperature of compositions containing this sulfide, will be primarly those of the base sulfide. The remaining essential sulfides all have their individual break-point temperatures and many of the compositions including. these sulfides may have temperature-resistance curves with a plurality of break points characteristic of each of the sulfide components which are present in substantial amounts.

The base sulfide is in general responsible for the electrical characteristics of the product. Any of the usual metal sulfides can be used at the base orsulfide in the practice of the present invention. For example, staple compositions have been prepared employing mixtures of an essential sulfide and one or more suldes of' the metals such as sodium, potassium, copper, silver, cadmium, platinum, mercury, lead, antimony, molybdenum, chromium, and nickel. It has also :been noted that the polysulfides of the present invention may be prepared from a mixture of two 1 or more of the sulfides which are referred to herein as the essential sulfldes in which case one of the group II metal sulfides may assume the role of the base sulfide component. Such stable products include for example calcium cadmium sulfide, calcium magnesium sulfide. zinc cadmium sulfide and mercury calcium sulfide.

Molybdenum sulfide For many applications and particularly to facilitate their manufacture, the compositions of the present invention also contain from about 0.03 to 10 percent, preferably less than 1 percent by weight, of molybdenum sulfide. It has been found specifically that molybdenum disulfide acts both as a lubricant and as a binder during the pressing y molybdenum'sulfide to function as a depolarizer,

i. e., to aid in the depolarization of the metal leads embedded in or in contact with the sintered mixture of sulfides.

Sulfur A 'further desirablev constituent of the mixed sulfides employed in the preparation of many of `the flementsiro'f thepresent vinvention :is-caramail' amount-'of freefsul'fur introduced into 'the mixe turesV prior'to pressingzin 'an :amount ranging fro'rn` 01.03 'to ashig'h as i 'l0 percent 'ibutgenerally not l"exceeding about V0.5 percentfby weight. .As willbep'ointed out m'orevfullyhereinaftenfafsmall amount y"of added/sulfur can-b'e'employed toaid in the 'stabilization'-oi the :resistance character istics of -=certa-in fof the resistance felements, particul'arly for D.-C. applications. LW hen i employed in vflarger amounts, rang-ingfup 4vto "10 Tpercent by weight, thesulfurialso serves toi-increase thesoverall'rresi'stance "of the Ielement'sfat lower temperatures.

Leads .provision vofleads icharacteri'zed by fafuseful .life

equaliltofthat:of"the'suliide.body per se. Hereto`fore4 considerable'troubl'e *has `rb een encountered in providing suitable ,electrical connections VVfor sulde 'resistance or semi-conducting materials. Many metals, fior example, silverfandfcopper,are readily attacked' by 'sulfur -and'this attack, 'which includes a Yconversion of the metallic silver .or copperto ra sulfide, 'may continue' until all of the metal is 'so consumed. The leads 4or ycontacts employed in-rthe ipracticefof the presentl invention possess the property "of reacting-only'to a limited extent with free sul-furlto f'form Von ythe surface ofthelead a thin, stable,Yprotectiveflmfof metal sulfide vrwhich apparently 'prevents `further reacti'onb'etween thametal andthe Ssulfuror sulii'e content of the-sulfide body. An example ofone cflthe leastfreactive lead.metalslis'platinum and this metal can'readily'fb'e used fior the'manufacture 'o f :electricallelementsifor @alternating-current applications. However, i-t1shouldfnot ordinarily be used .for accurate ldirect-current@applications due to the "fact thatwhen so usedzafcontinuous polarization fof'one of thepiatinuin leads cantake place over alperiod of `time 'Withaigradual increase inlv the-over-'all electrical resistance o'ftheelernent.

Examplesy o'fvsuitable'le'ads or contacts of the type lwhich are-sulfidedA readily-but only to a llimi-tedfextentfare those "ofirnolybdenum, chromium, nickelfan'd' nickel crfchromium-alloys. While such metals for lalloys, 'or metal vleads of -steel'or the like plated or coated with such metals or alloys, arev at f'first attacked fby thefsulfur,'the attackis not. arcontinuingfone as'. in fthe case 'of copper and theA electrical 'characteristics of fthe 'elements become constant upon the `rforrnationfof a stable, substantially indestructible and protective iilm of metal sulfldefon the surfaceof'the leadsl or contacts, i which sulfide layer'also feXhibitsa' constant and '-'stable relectrical resistance characteristic. Leads f ofor surfacedi-with molybdenum, nickel and 'afnickel alloy, e.A g.,"Nichrorne (80% 'l\Ti, 20% Cr) or Chromaloy (35% Ni, 15% Cr, bal Fe) Vand toa llesserext'ent aluminum leads Aare preferred. All of'thesehave the desired characteristics with regard tothe freedom'- offcomplete suliidationand disintegration; Tantalum'leads-a subject only to the-disadvantage "thatfit is quite `rdiiiicult to form-afsulfide layer -suilic'iently thick toact as amechfanical fbondbetween' the'lead andthe vsinteredfsulde mixture. VThis is valso vtrue of k'tungsten and y'thoriumecoated tungsten leads. While elements vin which one orfboth of vt'hel'eads are of rthis e'type -fare preferably f first lstabilized bya polarizationprocess `which `will be described more fully hereinafter;once they' are A'so stabilized, their electrical :characteristics for D.`"C. applications remain. constant'fortan indenniteperiodlf time and 'iaremot laffected .by Vssubffctinn stoubezeio.

temperatures vorto :elevated vtemperaturesiimlotv the sintering temperatures. Anyfof Jthesel'eads may falso be Yusedfor. elements .intendedjforJAs-C. applications 1 and `.in .such -cases, the polarization processrnaybe Aomitted or AVused :only=.tothe exten.t. necessary to obtain a good bonding of :the .leads tothe sintered sulfide.

Preparation off elements .In .the "preparation of vthe pnlysuliidesfof the present invention, the metal :.sulfides .are preferablyrpulverized'tofa nely .divided'statefandmixedl inthezidesired'proportions. Alternatelyravmixture:

orfall'oyr of .twoxmetals .including .a Zgroup II. metal may beconverted to fa `mixture of lsulii'des by treatment with hydrogen :sulfide tor sulfur zdiox- ,The ymixtures .which Imust contain .at l'east ide. two vmetal sulfldes, at least l'one of "which pis fone of the essentialvsulfides .are Upressed .to thedesred' form iorzshape. It has been;foundthatfapressurel ofthe Yord'erfof 20,000 ip. s. i. is desirable" iny forder to :obtain arproductofpa .density'such that :its resistance characteristics. are: una'iiected .by ffurther pressure. high `as40;000 p. `s.i. has Abeen found evenzmnre advantageous.

.During the `mixing and the 'pressing of. the suliides, icare should be exercised'to .see tliatlthe mixtures :remain ias free fasxpossible of ".various contaminants. .The :dies .and other equipment employed A1in-their Ymanufacture should Talso be.-

clean as it has been '.found that the 'presence :nf any significant amounts ofimpurities .makesait diflicult to prepareelements havingthesame electrical characteristics from batch to batch.

'The electrical connections?. in theform v.of leads.

break temperature, the sintering temperature should at' least' be abovethe'break temperature.

Fortmost. compositions containing a base sulfide" which in the proper proportions would give .a product having l`a 'break point temperature vv'below about 200 C., "the :preferred "sintering temperature ranges yfrom 200 to '40`0C. and :the'flements "aref'ordinaily `subjectedxto .fsuch vtemperature 'ifor y.about one hour; Higher sintering` temperatures'up to 1500o C. havesuc'cessfully been employed 'in the `preparation of vcertain elements and in 'some cases it Vhas beenfound 'that the break temperature of va given composition can'ibe .increased by sinteringthe .material atv a more:elevated"temperature While the sintering step :is zpreferred, lit may be 'omitted in' certain instancestas,"for'example, wherephysical"strength and a'sharp breakirpoint are. notrequired.

"Stbz'lz'aatz'onprocess "inthe `vpressed'or'pressed and sintcred condition, the elements .can satisfactorilyv be employed in'. most alternating-current applications. However, :in the pressed or as sintered condi-4 tion 2and;r.egard1ess of "Whether-i some vfree aslfur In many instances, pressure was added to the mixed suldes', ythe electrical conductivity of the sulfide elements is a combination of ionic and electronic action.

When a direct current is first passed through such elements including contacts embedded in or encasing part of the sulde body, there is a deposition of the free sulfur on the anode with a gradual building up of a layer of metal sulfide on the anode lead, for example, molybdenum sulde in the case of a molybdenum anode, and a corresponding change in the over-all resistance of the element. 'I'he result is a polarization of the element in the sense that its resistance in one direction is greater than that in the opposite direction. As it has been found that when the leads are of the type hereinbefore described the formation of the sulde on the anode ceases as soon as a protective nlm is formed on the lead using up the free (ionic) sulfur, one aspect of the present invention is the pre-conditioning or stabilization of such elements by a process which comprises alternately passing a direct current through the element rst in one direction and then in the reverse direction until the ionic sulfur is plated out on the surfaces of both metal contacts to form a stable and indestructible film of metal sulfide. Thus the stabilization process removes the sulfurand metal ions by plating them on the leads to form a stable sulfide or suldes. By using up all or almost all of the free ions in this process, there is obtained a stable element whose conductivity is only or mainly electronic rather than both ionic and electronic. This process can also be described either as involving a double polarization process in the sense that the leads of the element are both polarized, or as a depolarization process in the sense that the stabilized element is symmetrically conductive.

In carrying out the stabilization process, the current is reversed a suitable number of times during the stabilization treatment and the treatment is continued up to, for example, about 8 hours or until the resistance of the element is constant and the same in both directions. During the stabilization process, it has been found that the metal sulfide formed on the lead during the passage of a current in one directionis not removed or otherwise aifected by the reversal of the current so that after this polarization has been completed and the ionic sulfur plated out, the stabilized elements can be employed in any direct-current circuit without consideration'being given the question of direction of current flow.

. A further and important advantage of this stabilization treatment is that the migration of sulfur to the vicinity of the contacts and the formation of the metal sulde layer on the contact surfaces results in an anchoring of the contacts in or to the sintered sulfide mixture giving improved electrical and mechanical bonding and decreased contact resistance between the leads or contacts and the sulfide bodies.

In a preferred method of polarizing the electrode or electrodes in the manufacture of the stable sulfide elements, a D.C. current of reasonably high density, e. g. -200 amps/sq. in. at the smaller electrode 'is conducted through the element while the element is held at a temperature where the resistance is low or at a minimum value. Sulfur ions are conducted to the anode and deposit thereon to form a good bond between the electrode (anode) and the semi-conducting sulfide material. By reversing the polarity and making the other lead the anode before all-oi 8, the sulfur is plated out on the first lead, it is possible to build a sulfide film on the other lead also and cause a similarly good bond to form. By varying the time of operation in each polarity, it is possible to control the amount of sulfide film formed between each lead and the semi-conducting sulfide in a manner such that the resistance of the element is the same for both polarities.

The best time schedule for the treatment will depend upon the relative areas of the two elec-v trodes. For example, for an element in which the embedded surface areas of the two leads are about the same, the current is merely reversed a suitable number of times until the element has a constant resistance in both directions. On the other hand when the embedded area of one lead is greater than that of the other lead, it may be desirable during the first cycle of operations to proportion the times so that the lead with the greatest area is the anode for a comparatively longer time. For example, in processing an element such as shown in Fig. 4 in which the case is the larger lead or contact, the case is first made the anode for one hour and the lead having about one-tenth the area of the case is made the anode for ten minutes. Thereafter the current is reversed about every ten minutes for a total of eighthours, about every one-half hour for the next four hours and every hour for the next four hours. During this time the ammeter is checked and the current flowing in each p0- larity noted, and the times varied to get a product having the same resistance in both directions. When the element is fully depolarized, the current flowing should be 0f the same value in either polarity with the temperature constant. The total time is dependent also upon the amount of free (ionic) sulfur in the element.

When molbydenum disulfide is present in the mixture it appears to act as a depolarizer in the sense that it polarizes in the opposite direction from any Of the other metal suldes. For example, with nickel or nickel alloy leads, a molybdenum sulfide film forms on the lead in combination with the nickel sulfide of the lead material. The depolarizing effect of the molybdenum sulfide is a general one and is independent of the remaining sulfide or suliides in the composition.

The effect of molybdenum sulfide in thev polarization phenomena may best be understood by considering the electrical characteristics of two elements differing only in the fact that one contains a small amount of molybdenum sulfide and the other no molybdenum sulde. Two such elements would otherwise consist of about 25 percent calcium sulfide, percent silver sulfide and a small amount of free sulfur with molybdenum leads embedded at opposite ends of the elements.

When a D.C. current is passed for a short time in one direction through these two elements at a temperature of C., as in the first step of the stabilization process, and the resistance values of the elements on D.C. current thereafter measured in both directions at the elevated temperature, it will be found that the element containing no molybdenum sulfide will have a resistance of, for example, 1,000 ohms in the direction of flow of the current during the polarization step and will have a high resistance in the neighborhood of 100,000 ohms in the o'pposite polarity. On the other hand, at the same elevated temperature, the element containing molybdenum sulfide will be found to. have a re- 9, sistenceofabout 1,000.0hms. in the first. direction and.a..ver-y low resistance.of`,.,for example, .2 ohms intheoppositedirection.

If. the polarizing. current. is. thereafter passed through these twg elements inthe opposite. directon, no substantial change inthe resistance values of the element containing no molybdenum sulfidev willbe notedv On. the other hand, the element containing molybdenum sulfide,

when subsequently tested as hereinbefore described, Will show a substantial decrease in the 1,000l ohm resistance. at the elevated temperature until finally, after the current passing through. this; element hasbeen reversed` a suitable number of times, the-resistance of the molybdenum. sulflle-containingA element will be low and lnv the neighborhood of 2 ohms in either direction at any temperature above` its break point.

It should be. noted that before any polarization, the resistance values ofk both elements in bothv directions. are the same, .for example,y about 100G-ohms. at theelevatedtemperature of 180 C. and anywhere from 50,000v toA 100,000 ohms de pendingl upon. the free. (ionic)y sulfur content at room4 temperature. The room temperature resistance of. the molybdenum sulfide element is stabilized at. the. higher resistance of about 100,000 olimsiby the stabilization treatment.

In order that those skilled in the artbetter may understand how the present invention may be carried into effect, there is set forth hereinafter by way of illustrationv butV not by way of limitation a" detailed description ofV certain polysulfide elements within the scope of the invention.

Calcium silver sulfide element The calcium silver sulfide element is representative-ofthe polymet-al sulfide elements of the invention and the effectsof changing proportions, composition, sinter-ing temperatures, etc. will be described in detail in connection with this modiiication of the invention.

Calcium silver sulfide elements can be prepared which, dependingupon thecomposition, will exhibit either a continuous or substantially continuous decrease in resistance with increased temperature or a denitebreak point in the temperature-resistance curve. The nature of the curve as well as the resistance of the element of any given temperaturedepends primarily upon therelativeproportions of calcium and silver sulfide. 'I'he effects. of. varying the ratio of these two materials is illustrated in Fig. 6 of the-drawing.

The. curves plotted .in this figure were obtained by varying the proportions of cal'ciumand silver suliides of stabilizedelements prepared from mixtures containing about l percent molybdenum disulfide and about 1. percent sulfur. The elements were provided with either molybdenum or nickel chromium leads and', during the stabilization process, that sulfur which had not become chemically combined with the. calcium silver sulfide composition was plated out on. the leads in the form of .metal sulfide coating.

Asis' the. case with all of the other polysulflde elements, any of the leads described hereinbefore can be used, there beingvno noticeable difference in the resultant. resistance characteristics f the elements. Some leads, such as those of Chromaloy may require. a. slightly longer time for polarizatiomi. e., sulndation.

With.y reference to Fig. 6, it will be noted that compositions. containing. about 10 percent calcium. sulfide have a;temperature-resistance curve which is. substantiallyy a straight line. However,

10 as the calcium sulfide" content is increasedA the temperature-resistance curve bulges upwardly at temperatures below C. until at a composition which contains about 25 percent calcium sulfide, a curve is obtained which is characterized byj a maximum break point, i. e., a maximum drop in resistance over. a, relatively narrow temperature range. It is to be noted that during this entire increase in the calcium sulfide. content from 10 to about 25 percent, the resistance of the compositions at and above 175 C. isextremely small and for all practical purposes. zero. In fact, the temperature of 175" C..represents the break point o'f'those calcium silversulde velements havnga calcium sulfide content of. from about 20 tc13`0 percent by weight.

With further-increase in the calcium sulfide content, the low vtemperature-resistance Ofjthe element remains substantially the same asfth'e low temperature resistance of the compositions containing less than 30 percent calcium sulfide. The-high temperature resistance of such calcium sulfide krich mixtures gradually increases with increased calcium sulfide content and the temperature-resistance curve again approaches' a straight line.

In general, it appears advisable to employicalcium silver sulfide compositions containing at least 10 percent and not over 60 percentcalcium sulfide. Pressed mixtures of calcium and silver sulde containing less than 10 percent of the. calcium sulde are physically weak, While thosecontaining'over 60 percent have resistance valuesfof 1000 to 2000 ohms even atthe more elevatedzteina peratures.

With regard to the effect of thesulfur content of the stabilized elements, it appears that sulfur affects onlyy the low temperature-resistance values,.i. e., theelectrical resistance of the elementsY below the. break-pointl temperature. In Fie. 7 there isplotted av number of curves indi'- eating` roughly the change inthe low tempera"- ture-resistance values of calcium silver sulfide `elements containing 25% calcium sulfide, and from 0 to 10 percent added sulfur. In general, about 3 percent'sulfur is preferred in order to assure an ample supply 0f sulfur ions for-the depolarization of the leads and to form a coating of metal sulfide on these leads sufficient to provide a good mechanical and electrical bonding layer between the sulfide compositionand the leads.

It has also been found that the .resistance characteristics of the elements ofY the. presentinvention can be further modified without affecting theirv stability by the addition thereto of min'or proportions, for. example, up. to 30 percentvby Weight of powdered metal. For example, lthe addition of up to- 30 percent powdered silverto the-.calcium silver sulfide compositions results in a product, the resistance value of which decrease extremely rapidly with increased temperature. Thetemperature-resistance curve of a stabilized element containing 18 parts calcium sulfide-.80 parts silver sulfide, 18 partsmolybdenum sulfide, 1 part sulfur, and. 20 partsv powdered silver. is shown nFig. 8. The resistance value-of this eletment decreases toits lowest value ata tempera ture of about 175 C.

Many of the alloys of a metallic'. sulfideY and calcium sulfide aregreatly afectedby the-sintering temperature. This is particularly true of those which are sintered below` 600'@ C. Cal-- cium silver sulfide is in this group. When sin.- tered at 400, thecalcium silver sulfide has'a permanent and characteristic resistance curve'and shows very little, ifV anyaging effect after the first week.' However, when these compositions are sintered at only 300, there is an appreciable aging effect, but after about one week, standing at room temperature, such elements take on a permarient temperature-resistance characteristic which is the same as that obtained immediately by sintering the elements at about 400 C. When `the calcium silver sulfide elements are sintered at temperatures substantially above 400 C., the characteristics of the resistance curve are changed so that the resultant elements will not have as low a resistance value when heated above the break-point temperature as when sintered at 400 C. In general, in the preparation of most of the elements of this invention compounded to have a definite break point, it has been found desirable to sinter at a minimum temperature sube Calcium cadmium sulfide elements This is ari example of an element of the present invention comprising two of the essential sulfides, specifically, a mixture of cadmium sulfide and calcium sulfide. A composition containing 50 percent cadmium suliide, 40 percent calcium sulfide, percent molybdenum disulfide and 5 percent added sulfur exhibits a break point voi' 550 C. The cadmium calcium sulfide elements are preferably sintered at temperatures up to 1200 C. for maximum freedom from aging. In general, satisfactory elements can be obtained from compositions containing from 40 to 90 percent cadmium sulfide, to 60 percent calcium sulfide, 0.03 to 10 percent molybdenum disulfide and 0.03 to 10 percent excess sulfur. The sulfur may also be omitted particularly when it is not required for suliiding the leads.

Y Molybdenum calcium sulyide elements Particularly good elements are those in which the molybdenum disulfide is the base sulfide as Well as the binder and depolarizer. One such element is prepared from calcium sulfide, molybde'numV sulfide and, if desired, some added sulfur.

-These two or three materials are mixed Vin the usual manner, pressed at a high pressure, and thereafter sintered at a temperature up to about 1200 C.

The elements containing molybdenum sulfide as the base sulfide are representative of those in which the break-point temperature is affected by composition. For example, an element containing a sintered mixture of about 91.7 percent calcium sulfide and 8 percent molybdenum disulfide, and 0.3 percent sulfur exhibits a definite break in the temperature-resistance curve at about 250 C.V The break temperature increases with increased molybdenum disulfide content. An element containing 55 percent calcium sulfide, 35 percent molybdenum disulfide, and 10 percent sulfur exhibits a break point of about 350 C. The break point is independent of the sulfur or excess sulfur content within the range of 0 to 10 percent added sulfur and appears to be entirely dependent upon the relative proportions of the essential sulfide and molybdenum sulfide.

Preferably, the molybdenumy sulfidel is present in only a minor proportion. The reason for this is that when the content of molybdendum sulfide substantially exceeds 35 percent of the composition, the high temperature stability of the element is impaired and when an element containing more than 35 percent molybdenum sulfide is substantially overheated, a permanent change in the resistance characteristics in one direction will result. Elements containing mercury sulfide as the base sulfide are another example of elements in which the quantity of the base sulfide is kept low, however, for a different purpose. With a mercury sulfide element, a high dilution of the mercury sulfide is desirable to prevent spontaneous combustion or explosion of the mixture.

Other sulfldes Except for those compositions in which the break point temperature is dependent upon proportions of base sulfide as is the case with the molybdenum sulfide element, it is necessary to change the base suliide in order to obtain elements having diiferent break points. The following is the list of a number of suitable sulfides which have been used, the list including theV approximate break-point temperatures noted for mixtures of the designated sulfide with one of the essential suliides.

Approx. Break Point Temp.

Mixtures of chromium sulfide and calcium sulfide yield a product having a high and constant resistance at temperatures of from 0 C. to 900 C., i. e., products having a zero temperaturecoefficient of resistance. Platinum sulfide as the base sullide gives a material having a high negative coefficient below about C., a rather low coefficient from 150 to 280 C., and arather sharp break in the resistance values at about 280 C.

In general, it may be stated that the calcium sulfide (or its equivalent essential sulde) is usu ally employed in a minor proportion and preferably from 15 to 25 percent when a maximum break point is desired. Exceptions to this general statement are the molybdenum sulfide, mercury sulfide and chromium sulfide elements and the elements containing zinc sulfide as the essential sulfides.Y As to the last mentioned sulfide, best results have been noted with the silver Zinc suliides when the zinc sulre content was about 50 percent with satisfactory elements being obtained from mixtures containing as much as 70 percent Zinc sulde.

While the invention has been speciiically described with reference to sintered mixtures of one essential sulfide and one base sulfide (except for small amounts of molybdenum sulfide) ,it sto be understood that mixtures comprising more than one basesuliide and one or more essential sulfides are also within the scope of the invention. Such mixtures will in most cases exhibit temperaf ture-resistance curves which are, on the whole,

facciamo avcompos-ite offf the'l curvesl off' the'- separate-base sulfldes.

In'iall loff'thecompositions, the essential sulfide,

particularly thecalcium sulfide; finci'fir'ms. not

only to -iinpartstability to the mixturesbutal'so as a binder. It issforthisllatterreasonthatlan essential suliide content or" a-tleast' from` 3? to 5 sistance characteristics of'V` the= element-although from' tliemechanical standpoint', theanchori'ng offtlie lead'sas aresult of"the formation oithe sulfide; film thereon is also; ofi substantial importance. In addition, anyvfreev` or unattached sulfur ions. which have not-been. removed'` by evaporation orrcliemica'lly combined .With-the suldes= during the sintering- 'processl are. plated out during.V the'stabilizationprocess thereby using up any excesssulfur' Vso tliat'thev action.-` of` the 'elementis mainlyfelectronic.rather tlianacombination ofk ionic` and electronic; The termv excess sulfur as-used`r here refers notvso` much tothe total amount added'to` the-mixturepriorf to .pressing as: to that=.part whiclirhasinot entered into chemical combination. with 1 the` metal sulfldes .to

formeitlier.' a. metall.` sulfide-sulfur complex. or

a lower lmeta-1 sulii'd'e; 'in' other. Words, ther-.sulfur above-'thatrequiredI for tlie stoichiometrici-comipositions.

Iny this connection, the stabilizationzprocessiis '.not" limited-l tosulde resistance elements. but J is broadly-applicable to other' non-metallic; inor- 'tr'ius obtained? not. only a temperature fstab'leele'- ment but abonne-.Within which'. the leads; are v 'rmly. anchored orI secured'toi the 4res'i'stancemaryteriall as a'. result:v of the formationzof the stable compound or.rv compounds onutliez. leads. As in itl-ie caseY ofthe'sul'de elements'it may also.. be

desirable todeliberately have apresentfin the element composition a small amountloffanselement or com-pound which is .the sourceoffree. or' uncombined'. ions' that. are- Vfree to migrate tot-the vleads.during the polarization .treatment and'come bine therewith to. form a. stablecompound or layer. This may be accomplishedi'lbyf.adding: an

vion-providing'.N element: or' compoundv to..` thematerialf prior to the pressing or forming of:A the elementsor'in the. case',offthe,o.xides;..byfsintering or firingthe elementsin'anzoxygen or amoxyeen `ricii atmosphere.

Also Gduri'ngA the stabilization; process, any free .metal present in those elementsrprepared from mixtures inzcludirie'A added freelmetal`V reduces; the

other metallic sulildes. in. the mixtures? so that `the alloy becomesiless than'r a.- stoic'hiometricnnixf` turelcompound. VIn otherf-words with'i-add'ed silver Athealloy becomes' .f ai.si-lverfriclfiy alloyf The'fvelementsf of? the present invention.. Carr be used in any oi tlievarious'.applicationswl'iere a; thermally: responsive resistance element.' is-.required. They..y have thek added advantage over the previously known thermistor elements. oflbeing substantially.insensitive-to.changes1in voltage and of-'bein'g fastenougli in vresponse `to changes in temperature in the' neighborhoodv of'` their breakL .points to` be able to.` operate; alternating? current'relays directly.;

What' I claim asnewand desire'to..secureiby` Letters Patent of the United States-list l A stable' electric resistance uniti'. comprising a-body of resistance material consistingfessenftially offV a pressed mixture of metall sulides lirrcludingf (1li from 3r t'o 95per" cent of? af sulfide of a meta-l of group II of` the periodic table selected from the class` consisting of magnesium, calcium,y zinc, cadmium, barium and mercury,

'and (2) molybdenum sulfide and metal' leads electrically connected tol said bodyofresistance material. y

22 A stable resistancev unit composed off'a` body of" resistance material consisting essentially Y' of? a compressed', sintered mixture of metal sulildes including" at least 0.03 per cent molybdenum sulde and from l0 to 60 per cent oi at least one suliide4 of a` metal of group II ofthe periodic table and metal leadsffelectri'cally connected to saidl body of" resistance material.

3': A' stable', thermally sensitive-resistanceeunit composed ofr a'- body of resistance materia-ly consisting'of' a.' compressed,- sintered mixtureofa plurality of metal-subidos includingmolybdenum suliide, from 3 to 95 per'cent'of la sul'ilde' ofi-a. metal of group II'of theY periodi'otableand a substantial portion'of' a third metal suliide'- and metal leads electrically*connected-to"saidbodyof resistance material.

4. A'stable, thermally sensitive resistance-unit composed of a body of resistancel materiali com'- prising ai pressedV and sinteredrmixture of metal suliides essentiallyl includingjfrom aboutA 0.03"t"o 35A percentr molybdenum disulfide andv from 3 to 95` percent of asul'de of a metal of' group II ofthe periodic table and` metal contacts electrically connected' to saidl body of resistance material, at least one of said contacts being molybdenum.

A stable'polysulde' resistance' element com'- prsinga' body 'of resistance material consisting of a sintered` compressedI mixture of' at least' t'wo metal sulldes including from 10 to 95- percent off at'leaston'e sulfide of a' group, II' 'metal' and at least 0:03 percent'H molybdenumy sul'deA and metal contactsY electrically connected to said body;y at leastu one of said contacts being' a molybdenum contact embedded'in said body.

6. A stable, thermally sensitive resistance element. comprising a bodyv of," resistancev material consisting' ofA a compressed, sintered' mixture v'of metali sul'des including from about` 0.03 to I0 percent molybdenum sulfide, 10't'o' b'percentof a sullide of'a groupII metal and a substantial portionY of' a' third rnetalsuliideY and' inet'al contacts electrically connected to' said body.

"7'. A stable resistance element comprising" a body oil resistance .material consisting of. ai com*- pressedLsinter-ed. mixture. of' 0103 to l0 percent molybdenum disulde 40. to. percent cadmium sulfide and.. 10. to... 6.0y percent. calcium suliideA and metal. contacts. electricallyA connectedV to..l said body..

'Y 8. A stable thermistor consisting essentially of a body of resistance material consisting of a pressed, sintered mixture of a plurality of metal sulfides including molybdenum sulfide, 3 to 95 percent of a group II metal sulfide and silver sulfide and metal contacts electrically connected to said body.

9. A stable thermistor consisting essentially of -a body of resistance material consisting of a compressed sintered mixture of from 0.03 to 10 percent molybdenum disulde, 15 to 35 percent calcium suliide, balance substantially all silver sulfide and metal contacts electrically connected to said body.

10. A stable thermistor composed of a body of resistance material consisting,r of a pressed sintered mixture oi metal sulfides consisting of from 0.03 to l percent molybdenum sulfide, l5 to 25 percent calcium sulfide and. balance silver sulfide and metal contacts electrically connected to said body, at least one of said contacts being a molybdenum contact embedded in said body.

11. A stable resistance element comprising a body of resistance material consisting of a sintered mixture of .03 to l0 percent molybdenum suliide, l0 to 60 percent of a sulfide of a group II -metal, silver sulfide, and a small amount of sulfur and metal contacts electrically connected to said body.

12. A stable thermistor element comprising a body of resistance material consisting of a pressed, sintered mixture of at least 0.03 percent molybdenum sulfide, from to 60 percent of ,a sulde of a group II metal, silver suliide, a small amount of sulfur and a minor portion of powdered silver and metal contacts electrically connected to said body.

13. A stable thermistor element comprising a body of resistance material consisting of a pressed sintered mixture of at least 0.03 percent molybdenum suliide, 3 to 95 percent zinc sulfide and a third metal suliide and metal contacts electrically connected to said body.

14. A stable thermistor element comprising a body of resistance material consisting of a sintered mixture of at least 0.03 percent molybdenum sulfide, 3 to 95 percent cadmium sulfide `and. a third metal sulfide and metal contacts electrically connected to said body.

15. A stable thermistor comprising a body of resistance material consisting of a sintered mixture of at least 0.03 percent molybdenum sulde, 3 to 95 percent calcium sulde and a third metal sulde and metal contacts electrically connected to said body.

16. A stable resistance element comprising Va body of resistance material consisting of a `sintered mixture of molybdenum sulde, chromium sulfide and 3 to 95 percent calcium sulfide and metal contacts electrically connected to said body.

17. A stable resistance unit comprising a body of resistance material consisting essentially of a sintered mixture of metal suldes including molybdenum sulfide and 3 to 95 percent of a sulde of a group II metal and metal leads electrically connected with said resistance material.

18. A stable resistance unit comprising a body of resistance material consisting of a sintered vmixture of suldes including at least 0.03 percent molybdenum suliide and 3 to 95 percent of a sulfide of a group II metal and molybdenum contacts electrically connected to said body.

A19. A stable resistance unit comprising a body ofresistance material consisting of a compacted mixture of suliides including a small amount of y16 molybdenum sulfide and 3 tov95 percent of a sulfide of a, group II metal and metal leads electrically connected to said body including a molybdenum lead embedded in said body in electrical contact therewith, said lead having thereon a lm of molybdenum disulfide.

20. A stable resistor unit comprising a sintered, body of resistance material consisting of a sintered compressed mixture of metal sulfides including a small amount of molybdenum sulfide. 3 lto 95 percent of a sulde of a group II metal and. a third metal sulfide, and metal leads electrically connected with said body, at least one of said leads having a nickelous surface and having a sulfide layer on the surface thereof in contact with said body.

21. A stable resistance unit comprising a body of resistance material consisting of a sintered compressed mixture of metal suliides including a small amount of molybdenum sulfide, l0 to 60 percent of a sulde of a group II metal and a major portion of a third metal sulfide and metal leads including a suliide-coated metal lead electrically connected with said sulfide-coated body, said lead being an alloy of 35 percent nickel, 15 percent chromium, balance substantially iron.

22. A stable resistance unit comprising a sintered compacted body of resistance material consisting of a mixture of metal suliides including a small amount or" molybdenum sulfide, 3 to percent of a sulde of a group II metal and a major portion of a third metal sulfide and metal leads electrically connected with said body, one of said leads being of a nickel-chromium alloy and having a sulfide coating thereon.

23. The method of preparing a stable sulfide resistance element which comprises forming a mixture of at least two metal suliides including at least 0.03 percent molybdenum sulde and 3 to 95 percent of at least one sulfide of a group II metal, pressing said mixture into contact with a metal lead which Will react with sulfur to form a stable sulfide film thereon but which does not sulde to the point of disintegration of the lead, sintering the suliide mixture and thereafter passing a current through said mixture and lead to form a sulfide layer on said lead.

24. The method of obtaining a good mechanical and electrical connection between a polysuliide resistance element and an electrical contact element therefor which comprises pressing a powdered mixture of the suliides comprising the polysulflde element into contact with the contact element with a portion of one of said elements encompassing a portion of the other of said elements, heating the resultant product to sinter the polysuliide mixture and thereafter passing a direct current through the product to build up a suliide iilm on the surface of the contact element in engagement with the polysulde element.

25. The method of claim 24 wherein the mixture of suliide is pressed into contact with a contact element in the form of a wire having an end portion thereof embedded in the pressed mixture of suliides.

26. The method of claim 24 whereinthe mixture of suldes is pressed into contact with a contact element in the form of a cap enclosing a portion of the pressed mixture of sulfides.

27. The method ci obtaining a good mechanical and electrical connection between a resistance element comprising a sintered mixture of metal sulfides and a metal lead which comprises pressing a powdered mixture of the sulfldes including at least 0.03 percent molybdenum sulde and 3 to 95 percent of a sulfide of a group II metal and a small amount of sulfur into contact with a metal lead which has the property of reacting with sulfur to form a stable protective sulfide lm on the surface thereof, heating the resultant product to a temperature sufficient to sinter the sulfide mixture, and passing a direct current through said product to form on the surface of the lead in contact with the sintered sulfide mixture a layer of metal sulfide, said layer forming a good mechanical bond between said lead and said mixture.

28. The method of claim 27 in which the sintered sulfide mixture is held at a temperature at which the electrical resistance is low during passage of the direct current.

29. The method of claim 28 wherein the portion of the lead in contact With the sulfide mixture is molybdenum.

30. The method of claim 27 in which the sintered suliide mixture is held at a temperature at which the electrical resistance is loW during passage of the direct current and wherein the portion of the lead in contact with the sulfide mixture comprises nickel.

31. The method of claim 27 in which the sintered sulde mixture is held at a temperature at which the electrical resistance is low during passage of the direct current and wherein the portion of the lead in contact with the sulfide mixture is an alloy essentially containing nickel and chromium.

32. The method of claim 27 in which the sintered sulde mixture is held at a temperature at which the electrical resistance is low during passage of the direct current and wherein the portion of the lead in contact with the sulde mixture is an alloy consisting of nickel and chromium.

33. The method of stabilizing the resistance characteristics of a sulfide resistance element containing at least 0.03 percent molybdenum sulfide and 3 to 95 percent of a sulfide of a metal of 18 group I1 of the periodic table, which comprises passing a direct current through said element While holding the element at a temperature at Y which its electrical resistance is at a minimum value.

34. The method of forming a stable resistance element which comprises forming a mixture of a plurality of metal sulfides including molybdenum sulfide, 3 to 95 percent of a sulfide of a group II metal and a small amount of sulfur, pressing said mixture into contact with metal leads which will react with sulfur to form a stable surface layer of metal sulfide, sintering the pressed sulfide mixture and thereafter passing a direct current between said leads through the sintered mixture while the sintered sulde mixture is held at a, temperature above that at which its electrical resistance approaches a minimum, periodically reversing the direction of current flow, and continuing said process until the ionic sulfur remaining in the sintered mixture is deposited on said leads to form on the surfaces thereof sulfide films which mechanically and electricaly bond and connect said leads to the sintered sulfide mixture.

FREDERIC R. QUINN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 959,068 Phillips May 24, 1910 1,820,591 Andre Aug. 25, 1931 2,197,115 Randolph et al Apr. 16, 1940 FOREIGN PATENTS Number Country Date 144,608 Austria 1936 360,920 Great Britain 1931 367,790 Great Britain Feb. 15, 1932 701,172 France 1931 

1. A STABLE ELECTRIC RESISTANCE UNIT COMPRISING A BODY OF RESISTANCE MATERIAL CONSISTING ESSENTIALLY OF A PRESSED MIXTURE OF METAL SULFIDES INCLUDING (1) FROM 3 TO 95 PER CENT OF A SULFIDE OF A METAL OF GROUP II OF THE PERIODIC TABLE SELECTED FROM THE CLASS CONSISTING OF MAGNESIUM, CALCIUM, ZINC, CADMIUM, BARIUM AND MERCURY, AND (2) MOLYBDENUM SULFIDE AND METAL LEADS ELECTRICALLY CONNECTED TO SAID BODY OF RESISTANCE MATERIAL. 